1. Field of the Invention
This invention generally relates to cellular communications systems having mobile subscribers, and more particularly, to a method of controlling handover in the communications of cell site stations with a mobile unit in a TDMA cellular telephone system.
2. Description of the Related Art
Cellular communications systems use a series of sites, known as base stations, to communicate with mobile subscribers. A mobile telephone unit typically communicates with the closest base station, or the base station having the best signal quality. A cellular network often includes a set of many base stations, each base station communicating with mobile telephones in an area known as a cell. As a mobile subscriber moves from one cell to another, eventually the mobile telephone begins to communicate with a new base station covering the new area and ends communication with the old base station covering the old area in which the mobile subscriber was located. This process of beginning communication with a new base station and ending communication with an old base station is known as handoff or handover.
Various systems and techniques have been developed to address problems in handoff. Two examples of patents addressing handoff in cellular communication are U.S. Pat. No. 5,822,313 by Malek and U.S. Pat. No. 5,323,466 by Kojima, both of which are incorporated herein by reference in their entirety. In some systems, simultaneous communication links are established between the mobile unit and the base stations. In some prior systems, the handover process involved the use of dual transceivers in a mobile unit. Such a dual transceiver adds cost, size and power consumption, and reduced reliability to the mobile unit.
One embodiment of the invention is a mobile communication system including a plurality of base stations. The system includes a plurality of mobile telephones. Each mobile telephone communicates with at least a base station from the plurality of base stations using frames. A mobile telephone may establish communication with a first base station. If resources are available in a second base station, communication is established with the second base station. The communication occurs with the first and second base stations alternately, on different frames.
In one embodiment of the invention, communication may be established with a third base station. In such an embodiment, the communication occurs with the first, second, and third base stations alternately, on different frames. Communication may also be established with a plurality of base stations, in addition to the first and second base stations. The communication then occurs with the first, second, and plurality of base stations alternately, on different frames.
According to another embodiment of the invention, frames are passed alternately from the base stations, without selecting between the alternate frames based on signal quality. Also, the same frames from a telecommunications network may be passed to the first and second base stations, and this may occur in parallel at the same time.
According to various embodiments of the invention, communication with the second base station may be established based on signal quality. For example, communication may be established with the second base station if the signal quality of communication with the first base station is below a first particular level. Communication may be dropped with a base station if signal quality of communication with the base station is below a second particular level.
System capacity may be controlled by appropriate adjustments. For example, system capacity would be controlled by adjusting the combination of (a) a particular level of signal quality of communication with the first base station below which circuitry attempts to establish communication with the second base station and (b) a particular level of signal quality of communication with a base station below which the circuitry drops communication with the base station. Such combination may be adjusted automatically based on measured utilization of system resources, according to an embodiment of the invention.
The second base station may be selected based on signal quality. According to one embodiment of the invention, circuitry selects as the second base station a base station having the best signal quality between the base station and the mobile telephone, from among base stations other than the first base stations. According to another embodiment of the invention, the mobile telephone has a list of base stations ordered based on the signal quality between the base stations and mobile telephones, and the mobile telephone includes circuitry that selects the best second base station as the first available base station on the list. The list may be ordered based on received signal strength indicator (RSSI), in one embodiment of the invention, or frame error rate (FER), in another embodiment of the invention, or a combination of both.
Another embodiment of the invention is directed to a mobile telephone. The mobile telephone includes a single radio with a single synthesizer and circuitry that monitors the signal quality with various base stations. The mobile telephone includes circuitry that, when the mobile telephone is in communication with a first base station, causes the mobile telephone to request communications with a second base station based on the measured signal quality of the respective base stations. The circuitry causes the communication to occur with the first base station and the second base station alternately, on different frames.
An embodiment of the invention is directed to a base station. The base station comprises a radio capable of communication with mobile telephones. The base station comprises circuitry that establishes communication with a mobile telephone when the mobile telephone is in communication with another base station.
The circuitry receives alternate frames from the mobile telephones, which sends the frames alternately to the base station and another base station.
An embodiment of the invention is directed to a method of seamless handover in a telecommunications system. The method includes establishing communication with a second base station when a mobile telephone has established communication with a first base station and if resources are available in the second base station. Communication is caused to occur with the first and second base stations alternately, on different frames. According to one embodiment of the invention, communication is established with the second base station only if signal quality of communication with the first base station is below a first particular level. According to another embodiment of the invention, communication with a base station is dropped if signal quality of communication with the base station is below a second particular level.